The past two years have seen a paradigm shift in the management of metastatic melanoma. The development of BRAF^(R)^-specific inhibitors like vemurafenib has resulted in unprecedented response rates above 50%. However, the median duration of response is short (7 months) due to acquired resistance. To overcome this resistance there remains a need to further understand the underlying molecular signaling mechanisms. We will characterize the molecular details and network-wide properties of known resistance mechanisms and of not-yet-characterized mechanisms using mass spectrometry (MS)-based phosphoproteomics. Using this global approach we will quantitatively monitor signaling changes in thousands of proteins. We have established methods for interpreting the resultant system-wide data to uncover how the network of signaling events is modified upon the emergence of drug resistance, and to detect new signaling targets for therapy. Through our phosphoproteomic approach we have uncovered a mechanism of resistance mediated by a highly phosphorylated and truncated splice variant of BRAF. We will determine the molecular mechanism undertying this truncation-mediated resistance through mutational and functional studies. In these studies we will continue to use mass spectrometry as an unbiased approach to monitor the effects of our experimental perturbations. We will determine the role of phosphorylation through phospho-deficient mutants, and the role of BF?AF dimerization and protein interactions using mass spectrometry-based protein co-purification assays We have applied our approach to investigate the system-wide changes in signaling associated with the RTK-mediated mechanism of BRAF inhibitor resistance. Integrated genomic analyses have allowed us to prioritize several candidate signaling events as promoting the resistance-associated epithelial to mesenchyme-like (EMT-like) transition. Through additional phosphoprofiling we will refine this candidate list, then use mutational and functional studies to validate their signaling contnbution to resistance. With the investigators of this PPG we are building a comprehensive melanoma program. Our goal is to overcome BRAF Inhibitor resistance through an integrated approach that uses new technologies to learn from patient tumors and then translates this knowledge back to patient care. These studies have broader implications on other malignancies, since the BRAF^ mutation is present In 7% of all cancers. RELEVANCE (See instructions): Specifically targeting the mutations that drive melanomas has allowed for the development of therapies with greater efficiency and fewer side effects. Unfortunately, the complexity of cell survival almost inevitably provides alternative opportunities for disregulated cancer cells to become resistant to the initial therapy. By taking a system-wide approach looking at thousands of events simultaneously, we will delineate these intertwined mechanisms and provide a blueprint for using a cocktail of targeted drugs to prevent resistance.